Excimer lasers are the illumination sources of choice for the microlithographic industry. The use of high power lasers, for example, those with pulse energy densities (fluence) above 20 mJ/cm2, with pulse wavelengths below 250 nm (for example, 193 nm and below) can degrade the optics used in laser lithography systems. T. M. Stephen et al., in their article “Degradation of Vacuum Exposed SiO2 Laser Windows,” SPIE Vol. 1848, pp. 106-109 (1992), report on the surface degradation of fused silica in an Ar-ion laser. More recently, it has been noticed that there is optical degradation in high peak and average power 193 nm excimer lasers using materials made from substances other than silica.
Ionic materials such as crystals of MgF2, BaF2 and CaF2 are the materials of choice for excimer optical components due to their ultraviolet transparencies and their large band gap energies. Of these three materials, CaF2 is the preferred material due to its cubic crystal structure, performance, quality, cost, and relative abundance. However, the polished but uncoated surfaces of CaF2 optics are susceptible to degradation when exposed to powerful excimer lasers operating in the deep ultraviolet (“DUV”) range, for example at 248 and 193 nm and the vacuum ultraviolet (“VUV”) range, for example at 157 nm. For lasers operating at 193 nm, 2-9 KHz, with pulse energy densities of 20-80 mJ/cm2, the surfaces of the optical elements made from these ionic materials are known to fail after only a few million laser pulses. In other applications, for example medical lasers, alternate operating parameters could exist such as 193 nm laser fluences of 200 mJ/cm2-1000 mJ/cm2 (very high fluences) and very low repetition rate (for example 10-100 Hz) that may also result in the accelerated failure of such optical elements. The laser damage is thought to be the result of fluorine migration from the crystal optic interior or bulk to the surface where the fluorine is lost to the atmosphere. The loss of fluorine from the CaF2 crystal optic results in the formation of F centers which can then combine to form Ca colloids near the surface and within the bulk. These Ca colloids subsequently increase scatter and heating of the optical element, with eventual catastrophic failure. U.S. Pat. No. 6,466,365 (the '365 patent) describes a method of protecting metal fluoride surfaces, such as of CaF2 optics, from surface degradation by use of a vacuum deposited coating, for example, a silicon oxyfluoride material. While coatings may be sufficient to address surface damage, the microlithographic industry constantly demands greater performance from excimer sources, and consequently from optical components used in connection with excimer laser based systems. Therefore, the laser durability of the bulk material, CaF2, must also be improved by limiting the formation of Ca colloids that result in the eventual failure of the optical element. This solution will either eliminate the problem or greatly extend the bulk durability and consequently the length of time that existing and future optical elements can be used without having to be replaced.
Solutions to the issue of optical element lifetime involving the use of other optical materials, such as MgF2, have been considered. However, it is believed that such materials will also experience degradation similar to that of CaF2 with time, leading to the same requirement; i.e. that the expensive windows be replaced. It is further believed that the degradation problems of CaF2, MgF2, and other fluoride optical materials will be exacerbated with the advent of laser systems operating at wavelengths below 193 nm. Thus, identifying a method to increase the laser durability of the CaF2 bulk appears to be the most straightforward method of achieving the industry demands for improved laser performance.